1. Field of the Invention
The present invention relates to a network interconnection apparatus, a network node apparatus, and a packet transfer method suitable for an internet environment formed by connection-oriented networks.
2. Description of the Background Art
In recent years, due to increasing demands for a variety of communications such as an image communication and a high speed data communication, there is an eager expectation for a realization of the B-ISDN (Broadband-Integrated Service Digital Network) in order to provide highly efficient and flexible communication services, and the ATM (Asynchronous Transfer Mode) exchange scheme is considered as a prospective scheme for actually realizing the B-ISDN. This ATM exchange scheme is a scheme for realizing the communication service by loading data into a fixed length packet called cell regardless of the attributes of the data, and using this cell as a unit of exchange. The ATM communication technique is now studied extensively as a platform for realizing a multi-media communication and a high speed, large capacity communication, in a field of the public network (B-ISDN) as well as in a field of the LAN (Local Area Network).
Now, in the conventional LAN environment such as that of the Ethernet, the inter-LAN connection, i.e., the inter-networking among the LANs, has been realized by providing a router between each adjacent LANs. The main function of this router is the routing processing for the datagram transmission over the LANs, by processing up to the layer 3 (network layer) in the OSI (Open System Interconnection) protocol layer stack. Namely, for the datagram to be transmitted over two LANs, the datagram must be brought up to the layer 3 by the router to analyze the destination network layer address there, and then delivered to the destination LAN according to the result of this analysis. Here, it should be noted that, this “router” is often also referred as “gate-way” in the field of the computer communication, but a term “gate-way” is formally defined as an element which carries out the processing up to the layer 7 in the OSI, so that “gate-way” is actually different from the “router” strictly speaking.
There is also an element called “bridge” which has the similar function as the router in realizing the inter-LAN connection. In this bridge, in contrast to the router which determines the destination LAN by analyzing the destination network layer address, the destination LAN is determined by analyzing the datalink layer address (MAC address). Namely, the bridge realizes the inter-LAN connection by analyzing the destination MAC address of the datagram and passing the datagram through to another LAN when the obtained MAC address is not destined within its own LAN. Thus, in the bridge, only the filtering of data is carried out and the functions of the network layer are not realized.
Furthermore, there is also a similar element called “brouter” which has the function of the router as well as the function of the bridge. Namely, this brouter functions as the router for the predetermined network layer protocol and as the bridge for all the other protocols. In other words, in the brouter, those that can be handled by the router are all handled by the router function, while those that cannot be handled by the router are handled by the bridge function.
The function of the router, bridge, or brouter has been usually realized by the workstation (WS). Namely, the function of the router, bridge, or brouter has been realized as the CPU provided within the WS carries out the address analysis and transmits the datagram to the allocated physical port.
Now, one of the features of the ATM communication scheme is its high speed operation realized by the hardware switching of the ATM cells. That is, in the ATM communication scheme, the virtual connection (VC) or the virtual path (VP) is set up end-to-end, while the data to be exchanged end-to-end is loaded in the payload section of the ATM cell, and the ATM cell is exchanged and transmitted up to the destination terminal by the hardware switching operation alone without the intervention of the software operation, where the hardware switching operation is carried out by the ATM switch according to the VPI/VCI (Virtual Path Identifier/Virtual Channel Identifier) or the value of the other field such as PT in the ATM cell header which is contained in the ATM cell header.
From this point of view, the ATM communication scheme can be considered as a communication scheme which achieves its high-speed operation by referring only to the ATM cell headers, setting up the virtual connections/paths end-to-end according to the ATM cell header values, and carrying out the hardware switching. In a case of applying this ATM communication scheme to the LAN, it is considered that the communications between the terminals in the LAN are realized by the communications through the ATM-VC/ATM-VP as described above, and it is possible to expect a drastic speed up and capacity increase for the communications between the terminals.
However, in a case of the ATM-LAN, when the inter-LAN communication is to be carried out, the network layer transfer is carried out at the router between the LANs, so that the inter-LAN communication has a problem in that the speed and the capacity of the communication can be considerably lowered compared with the communication within the LAN. In addition, due to the processing overhead at the network, there has also been a problem that the probability for the occurrence of the congestion becomes high as a result of the lowering of the transfer speed and the limitation of the processing speed.
On the other hand, the inter-networking scheme used in the conventional data network is the connection-less scheme, in which the data unit (network layer service data unit) transmitted to the router by using the datalink is applied with the OSI layer 3 processing at the router, and then the relaying of the data unit is carried out. As described above, the bridge for carrying out the filtering of the data unit of the datalink layer only carries out the filtering of the data unit and the functions of the network layer are not realized.
Recently, there are propositions for the inter-networking not only in the connection-less mode but also in the connection-oriented mode as well. In short, these propositions introduce the concept of the connection into the inter-networking at the datalink level or the inter-networking at the network layer level. The ATM is a prime example of the former type, while the ST-II recently proposed as the connection-oriented network layer protocol is an example of the latter type.
In the ATM, the reservation of the communication resource is made at the datalink level, while in the ST-II, the reservation of the communication resource is made at the network layer level. In either case, the set up of the connection is carried out before the communication, so that they are basically the connection-oriented schemes. Here, for the connection-less communication, the processing of the network layer is carried out at the server (CLS, router, etc.) of the connection-less communication, whereas for the connection-oriented communication, the header information rewriting table necessary for the reservation of the communication resource and the relaying is set up before the start of the communication, and then the actual data transfer is handled by the relaying at the datalink level.
However, the currently available usual applications realize the connection-oriented communication on the connection-less service, such that the data unit is transferred by using the connection-less communication mode even for the connection-oriented communication. In the connection-less mode, a plurality of processings of the network layer are carried out, and it is common to re-construct the data unit of the network layer in this case. Consequently, even in the application which is designed to realize a high speed communication by carrying out the relaying at the datalink level, there has been a problem that the probability for the occurrence of the congestion becomes high as a result of the lowering of the transfer speed and the limitation of the processing speed due to the processing overhead at the network.